Everyday social activities such as toy play with parents are the context for learning as it unfolds in real time. A well coordinated child-caregiver interaction seems likely to lead to better learning while a decoupled or non-coordinated interaction may disrupt learning and development. Both parent and child play an active role in early communication and word learning as children signals their choices of communication and also determine what environmental information is most relevant to their own developmental needs, and as parents react to those signals in a sensitive manner and provide relevant information to ease the challenge of children's matching linguistic symbols to their referents. The goal of the proposed research is to achieve a deeper understanding of the sensorimotor basis of early social coordination and its potentially critical roles in later language learning an other development milestones. Toward this goal, the proposed research has three key components: 1) a set of longitudinal and cross-sectional experiments will collect multiple streams of sensorimotor data from child-parent toy play to discover fine-grained patterns characteristic of early developmental changes in child-parent social interactions which will provide new evidence on the developmental origins of these skills; 2) we will link sensorimotor dynamics in child-parent interaction with standardized, highly reliable behavioral measures that have been widely used, with the goal to understand how children's moment-to-moment social interactions with social partners may build generalizable word learning skills; 3) we will link social coordination in toy play with parental responsiveness and individual differences in development milestones, which will provide deeper insights into the consequential and longer-term role of early parent-child interactions in developmental process.